1. Field of Invention
The present invention relates to an apparatus and method for data communication through a telephone communication network. More particularly, the present invention relates to an apparatus and method for highly reliable data communication by changing a communication mode adaptively to telephone line condition using a simplified configuration.
2. Description of the Related Art
Traditional communications equipment for data communication through a telephone communication network consists of a modem for modulating and demodulating digital data to transmit the data through analog communication lines constituting the telephone communication network. This kind of modem conforms to Standards recommended by International Telecommunication Union Telecommunication Standardization Sector (ITU-T). Among the standards, V.34 allows modems to transmit data at up to 28,800 bit/sec maximum. And although V.34 enables data transmission at a higher rate than other Standards, such as, V.17, V.29, V.27, among others, V.34 has lower communication reliability than other Standards under abnormal line conditions, such as the presence of noise in the communication line.
Specifically, transmitting and receiving terminal modems, transmitter, and receiver communicate with each other using the V.34 start-up procedure based on V.8 under the V.34 mode. If the V.34 start-up procedure has been successful, data communications are continued under the V.34 mode. Conversely, the communication mode changes to the V.17 mode if the V.34 start-up procedure fails (refer to ITU-T T.30).
However, the V.34 start-up procedure may successfully complete, and communications may proceed under the V.34 mode, despite an irregular channel condition subject to noise. In this situation, a command may not be successfully received from the transmitter's modem, or a Resync/Interpage negotiation may fail due to the abnormal line condition.
To solve these problems, Japanese Patent First Publication No. 2000-101810, which is hereby incorporated by reference, proposes a communication device capable of preventing occurrence of a communication error by selecting an optimum communication mode in response to line conditions. Referring to this publication, the signal-to-noise ratio (SNR) of a line is calculated by using the noise voltage detected by a voltage detection section during a silence period of the line, and a voltage of an ANSam signal detected by the voltage detection section when receiving a protocol signal. The calculated SNR is compared with a predetermined value. As a result, if it is determined that the line condition is appropriate for the V.34 communication mode, communication is conducted in the V.34 mode. If not, communication is conducted in a V.17 communication mode, or lower.
The communication device taught by Japanese Patent First Publication No. 2000-101810 requires an AC voltage detection circuit to detect an AC voltage on the line when calculating the SNR, and thus, it is structurally complicated. Further, the SNR can be calculated after measuring the voltage twice, that is, once during the silence period, and again when receiving a protocol signal, thereby making this technique a complicated process.